There have been developed a number of trench-type power MOS transistors and IGBTs (Insulated Gate Bipolar Transistors) capable of on-resistance reduction, speed-up, cell pitch reduction and the like. To reduce the substrate resistance or to increase the density of integration of unit elements, there have also been proposed three-dimensional trench gate power MOS transistors and IGBTs structured three-dimensionally.
The conventionally proposed three-dimensional trench gate power MOS transistor structure or three-dimensional trench gate IGBT structure requires complex control of internal carriers attributable to the complicated element structure of the three-dimensional trench gate. As a result, there arise problems such as reduction in breakdown tolerance of the element, reduction in an ASO (Area of Safe Operation: secondary breakdown tolerance of the element) and difficult control of a threshold voltage (Vth).